Precision resistance and method of making it



1951 B. 8. Soon" El AL 2,570,163

PRECISION RESISTANCE AND METHOD OF MAKING IT Filed Dec. 28, 1948 Fig.2. Iii

' :II-I-lililalllii I Inventors: Benjamin B. Scott. Almy D.Coggeshall,

b @M/m Them Attorney.

Patented Oct. 2, 1951 UNITED STATES PATENT OFFICE PRECISION RESISTANCE AND METHOD OF MAKING IT Benjamin B. Scott and Almy D. Coggeshall, Schenectady, N. Y., assignors to General Electric Company, a corporation of New York Application December 28, 1948, Serial No. 67,586

4 Claims. (01. 201-63) Our invention relates to precision wound electrical resistances having intermediate taps and to a method of making them.

The object of our invention is to make tap connections at accurately des nated points along the winding lengths that have the best possible electrical and mechanical stability and low resistance despite intense vibration over a wide range of frequencies.

One of the ways previously used to attach tap connections to precision resistance cards, involved spreading of wire turns on each side of the cards at tapping positions and drilling small holes in the card through which fine enameled wires were drawn and soldered directly to the turns to be tapped. Another way comprised the use of contact pieces carried by insulating pieces attached to the cards so as to provide solderless spring contact connections. In a third method copper connection strips were soldered to tap turns, insulated from adjacent turns by small insulating strips, slipped under part of the winding above and below the tap turns. The tap assembly of this third method was then fastened to the resistance cards by conventional type bonding agents.

None of the tap making methods outlined above is able to produce connections that meet the precision electrical characteristics or the mechanical strength characteristics of a precision resistance embodying our invention.

In accordance with our invention, during the winding of a resistance, the tap point, determined by measurement of resistance, from the winding start point, is marked by a paper marking strip inserted beneath the wire and held by the tap turn until winding is completed. Insulating material is inserted beneath the tap point and a copper connection strip is soldered to the tap point. A resin impregnated insulation sheet is applied over the outside of the connection and the whole resistance is subjected to a heat-pressure curing treatment to secure a smooth even outside surface on the resistance. Resistances made by this process are accurate, strong, and have tap wire connections as sturdy as the coils of the resistance, that are positioned with single turn accuracy.

For a more complete understanding of our invention, reference is made to the accompanying drawing of which Fig. 1 shows the position of a marking strip placed beneath one wire of a wound resistance placed there during winding to indicate the exact position of the tap; Fig. 2 shows the marking strip removed and a square piece of cellophane on top of a piece of insulating material centered beneath the marked turn; in Fig. 3 the cellophane square has been removed and two dams have been attached covering the tap turn to prevent external dislodgement of the tap turn; Fig. 4 shows a copper strip on top or an insulating strip inserted beneath the wire between the dams and soldered to the wire, and an insulating square covering the whole connection; Fig. 5 shows the wire turn and a flexible wire lead soldered to a copper strip inserted beneath the wire turn, an insulating petticoat beneath the flexible lead and a piece of insulation covering the entire connection.

Referring to the drawings, the resistance i is formed by winding wire 2 on the turn supporting card 3 on a winding machine (not shown). The machine is stopped at the calculated tap turn, as determined by a scale attached to the machine measuring along the length of the supporting card. The exact position of the tap in this turn is then determined by Wheatstone bridge measurement of the wire resistance from the starting point or first terminal end of the resistance. The tap point is marked by a narrow piece of paper 4 called a marking strip which is positioned be neath the wire to index the position of the selected tap point until aiter the remainder or the resistance is wound. The winding machine il restarted with the tap turn holding the marking strip in place leaving the two ends of the marking strip projecting straight out from the resistance body as shown in Fig. 1.

When the winding is completed, the tap turt 5 is lifted slightly from the card face by mean: of the marking strip and a flat reinforcing stri; 6 approximately x /1," is centered beneath the tap turn. This strip may be of electricalli insulating paper, glass or rayon cloth or other suitable insulation material. The marking strip 4 is removed and a strip 1 of cellophane approximately /4" square is centered on the reinforcing strip 6 beneath the tap turn 5 at the tap point to prevent the wire from sticking to the reinforcing strip in the subsequent heat pressure operation to which the wound resistance is next subjected. A coating of a thermosetting resin is applied to the resistance by dipping in a solution of the resin and the coating is heat-pressure cured be tween heated platens to bond the wire turns to the faces and edges of the supporting card, after which glass cloth damming strips 9 and I9, approximately x x 0.005" thick treated with thermosetting resin are centered over the tap turn 5 along both edges of the reinforcing strip and the resistance is similarly resin dipped and heat-pressure treated to set or cure the resin into a homogeneous bonding medium in order to maintain the turns in exact position during the succeeding steps of manufacture. The cellophane pieces I are removed and the tap turn I is lifted and stripped of resin between the damrning strips 9 and H) which are used to prevent disicdgement of the tap turn outside of the damrning strips and consequent turn to turn shortcircuiting.

A copper connection strip ll centered upon strip of thermosetting resin impregnated glass cloth [2 is inserted beneath the tap turn 5 be tween the damming strips 9 and I and the connection strip is soldered to the turn at the point l3. After a cover piece 13' of thermosetting resin treated glass cloth square is centered over the soldered connection, the resistance is again resin dipped and heat pressure treated to incorporate the cover in the entire resistance.

The various thermosetting resin heat bonding treatments here outlined have preserved, in the finished resistances, the accuracy due to the careful positioning of the wire turns achieved in the winding process and the careful location of tap points despite further operations upon the resistances before completion. The finished resistance, in addition to having a high order of accuracy, is of very good rugged construction as compared to other resistances similarly employed. In addition, the tap connections are very nearly as strong as the rest of the resistance. Thermo-setting resin heat-bonding treatments similar to those described above, and the resistors produced thereby form a portion of the subject matter described and claimed in U. S. Patent 2,534,994 issued to Benjamin B. Scott, George F. Green, Almy D. Coggeshall on December 10, 1953 and assigned to the same assignee as this application.

Where flexible wire tap connections to cards are required, the same process is used except that small copper connection strips (Fig. 5) are soldered to the tap turn 5 at 13 and to the flexible wire 14 at (6. Where there is danger of short circuiting flexible wire connections to card turns in the vicinity of the tap point thermosetting resin treated glass petticoats I! and is are placed under and over the flexible wire and are heatpressure treated for a portion of their lengths to provide electrical insulation.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A tapped precision resistance capable of being brush contacted while maintaining desired gradient resistance values comprising a support of insulating material, a resistance wire wound in a plurality of turns on said support, a first strip of uncured thermosetting resin-impregnated insulation material positioned beneath a portion oi one of said turns and extending over other or said turns, two damming strips of uncured thermosetting resin-impregnated insulating material spaced on said first sheet and across said one turn, a coating of resin material applied to said resistance and heat-pressure treated to bond said turns in spaced relation on said sup port and to anchor opposite ends of said portion said one turn, an electric conductor positioned on said first strip and electrically connected to said portion of said one turn, a second strip of uncured thermosetting resin-impregnated insulation material positioned under said conductor and tap connection and between said damming strips,

third strip of uncured thermosetting resinimpregnated insulation material extending over sa'd damming strips and said electric connection to provide for securing said strips together and to said turns and support without dislodging said turns by heat-pressure treating said resistor to form one homogeneous mass, and a brush path formed across said turns.

2. A tapped precision resistance capable of being brush contacted while maintaining desired gradient resistance values comprising a support of insulating material, a resistance wire wound in a plurality of turns on said support, a first strip of uncured thermosetting resin-impregnated 1nsulation material positioned beneath a portion of one of said turns and extending over other 0! said turns, two damming strips of uncured thermosetting resin-impregnated insulating material spaced on said first strip and across said one turn. a coating of resin material applied to said resistance and heat-pressure treated to bond said turns in'spaced relation on said support and to anchor opposite ends of said portion of said one turn, a flexible conductor electrically connected to said portion of said one turn, a first and a second petticoat strip of uncured thermosetting resin-impregnated insulation material positioned the flrst beneath said flexible conductor and the second above said conductor and covering said electrical connection andsaid damming strips to provide for securing said strips together and to said turns and support without dislodging said turns by heat-pressure treating said resistor to form one homogeneous mass, and a brush path formed across said turns.

3. The method of making a tapped resistance provided with turns of raistance wire wolnd on a support and including at least one intermediate tap turn having a conductor connected thereto, which comprises determining the tap turn and the position of the electric connection point on said turn during the winding of the wire on the support, marking said connection point by inserting a marker strip of flexible material beneath said turn at said connection point, winding the remainder of the wire on said support, raising said tap turn from said support by means of said marker strip, inserting a first sheet of uncured thermosetting resin-impregnated insulating material beneath a portion 0! said tap turn at mid tap point and extending over others of said turm, placing damming strips of uncured thermooetting resin-impregnated insulating material on said first strip on opposite sides of said connection point and extending each over said tap turn, applying a coating of resin material to the resistance and heat-pressure treating the resistance to bond said turns in spaced relation on said support and to anchor opposite ends of said portion of said one turn, inserting a strip electric conductor between said sheet and said tap turn, making an electric connection between said conductor and said tap point, inserting a second sheet (I! uncured thermosetting resin-impregnated insulation material between said damming strips and under said conductor, placing a cover sheet of uncured thermosetting resin-impregnated insulating material over said electric connection. securing said cover sheet, said first and second sheets and said damming strips in one homogeneous mass to said resistor by one heat-pressure treatment of said resistor, sheets, and strum. and removing said resin from a portion of sold turns to form a brush path.

4. The method of making a tapped resistance provided with turns of resistance wire wound on a support and including at least one intermediate tap turn having a conductor connected thereto, which comprises determining the tap turn and the position or theelectric connection point on said turn during the winding of the wire on the support, marking said connection point by inserting a marker strip 01' flexible material beneath said turn at said connection point, winding the remainder of the wire on said support, raising said tap turn from said support by means of said marker strip, inserting a first sheet of uncured thermosetting resin-impregnated insulating material beneath a portion of said tap turn at said tap point and extending over others of said turns, placing damming strips of uncured thermosetting resin-impregnated insulating material on said first strip on opposite sides of said connection point and extending each over said tap turn, applying a coating 01' resin material to the resistance and heat-pressure treating the r to bond said turns in spaced relation on said support and to anchor opposite ends of said portion of said one turn, inserting a strip electric conductor between said sheet and said tap turn, making an electric connection between said conductor and said tap point, electrically connecting a flexible conductor to said strip conductor, inserting a second sheet of uncured thermosetting resin-impregnated insulation material between 6 said damming strips and under said strip conductor, placing a first and second petticoat sheet 01' thermosetting resin-impregnated insulation material the; first under said flexible conductor and the second over said flexible conductor and said electric connections and damming strips, securing said sheets and strips in one homogeneous mass to said resistor by one heat-pressure treatment of said sheets and strips, and removing said resin from a portion of said turns to form a.

brush path.

BENJAMIN B. 80011. ALMY D. COGGESHAIL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,627,213 Stone May 3,1927 1,930,932 Freyman Oct. 17, 1933 2,051,517 Creager Aug. 18, 1936 2,163,797 Mucher June 27, 1939 2,406,383 Kindermann Aug. 27, 1946 

